This invention relates to a top plate for use in a cooker and, in particular, to a top plate having enamel coatings formed on a low-expansion crystallized glass of deep color.
Recently, demand is increasing for an electric cooker or an electromagnetic cooker provided with a heating element, such as a halogen heater, under the top plate because of its safety.
As a top plate of the cooker, a low-expansion crystallized glass plate is used with a low coefficient of thermal expansion of about -5 to 30.times.10.sup.-7 /.degree.C. within a temperature range between 30.degree. and 750.degree. C.
In Japanese Patent Publication No. 3-9056 (JP-B-3-9056), a low-expansion crystallized glass is disclosed which has a deep color and used for the top plate. The crystallized glass consists, by weight, of 60 to 70% SiO.sub.2, 14 to 28% Al.sub.2 O.sub.3, 2.5 to 5.5% Li.sub.2 O, 0.1 to 0.9% MgO, 0.1 to 0.9% ZnO, 3 to 6% TiO.sub.2, 0.03 to 0.5% V.sub.2 O.sub.5, 0.1 to 1% Na.sub.2 O, 0.1 to 1% K.sub.2 O, 0 to 1% CaO, 0 to 2% BaO, and 0 to 3% PbO. The crystallized glass contains precipitated solid solution crystal of .beta.-quartz and has a black appearance. Such a deep color, low-expansion crystallized glass plate is excellent in strength and thermal shock resistance, and has a high transmittance for an infrared ray but a low transmittance for a visible light.
Accordingly, when the heating element is working, the heating element can be confirmed through the top plate. On the other hand, when the heating element is not working, the heating element is not seen through the top plate.
In order to paint a surface of the low-expansion crystallized glass plate of the type described, an enamel frit composition is widely used which comprises glass powder typically used as an enamel or glaze for a ceramic article, and a coloring pigment. An enamel coating is produced by mixing the glass powder and the coloring pigment to form a paste, applying the paste on the surface of the crystallized glass plate by screen printing, and burning the paste at a predetermined temperature.
In recent years, a light color, such as beige, is preferred as a color tone of a panel and a frame of the cooker. In order to harmonize the color tone, it is desired that the enamel coating of the top plate has a light color.
However, in case where the enamel coating of such a light color is formed on the surface of the above-mentioned low-expansion crystallized glass plate of a deep color, it is difficult to obtain a desired color tone because the color tone of the crystallized glass plate is seen through the enamel coating.
In order to avoid that the color tone of the low-expansion crystallized glass plate of a deep color is seen through the enamel coating, the thickness of the enamel coating is increased and/or the coloring pigment is mixed at a high mixing ratio. However, such approach causes various problem.
Specifically, when the thickness of the enamel coating is increased, abrasion resistance is deteriorated. Accordingly, when the cooker is used, the enamel coating is easily damaged by friction with a pan or pot. Furthermore, if a large difference exists in the coefficient of thermal expansion between the low-expansion crystallized glass plate and the enamel coating, an increase in thickness of the enamel coating results in frequent occurrence of cracks and peel-off and is, therefore, unfavorable.
On the other hand, in case where the coloring pigment is mixed at a high mixing ratio, the abrasion resistance of the enamel coating is deteriorated and the acid resistance is also decreased. While the cooker is used for a long period of time, the enamel coating is readily damaged, for example, by spilling of food during boiling.